"There's one bit where we're sitting around a tea table, in spacesuits, and there's Ice Warriors, and there's us, and we're on Mars... I was just looking around, thinking, 'In no other show would this ever happen!' It's just mad!

"But I really liked them. I thought they were cool... and very complex as well, which is always good for a monster. Nothing's just straightforward evil, as well."

The new series of Doctor Who also features some top-quality guest stars – from Ralf Little to Mina Anwar to the returning John Simm – but perhaps Mackie's favourite was David Suchet, who appears in episode 4.

"David Suchet, that was wonderful – I mean, I've watched so much Poirot with my mum!" Mackie giggled. "So I was like, this is amazing, I can't believe I'm working with him!

"He's such a wonderful man, as well – one of the gentlest, funniest, kindest people I've ever come across – and watching him work is phenomenal. I've never seen anything like it, it was amazing."

Doctor Who returns to BBC One on Saturday (April 15) at 7.20pm on BBC One.